1. Field of the Invention
The present invention relates to transmission controls, and more particularly to hydraulically operated control arrangements for use with transmissions in tractors or other agricultural vehicles.
2. History of the Prior Art
It is well known to control transmissions such as those used for tractors and other agricultural vehicles using hydraulically operated controls for monitoring various conditions within the transmission and for selectively applying pressurized hydraulic fluid to different portions of the transmission to perform various different functions. The control may, for example, control the application of hydraulic fluid to the clutches within the planetary section of the transmission to effect shifting between direct drive and underdrive conditions where the clutches are hydraulically operated. The control may also be used to selectively control the application of hydraulic fluid to such things as a power takeoff shaft and a mechanical front wheel drive where these portions of the transmission are also hydraulically operated.
Most conventional hydraulically operated transmission controls function satisfactorily for many applications, so long as full or optimum hydraulic fluid pressure is available. However, if hydraulic fluid pressure is reduced due to leakage or malfunction in one or more parts of the system or is completely lost such as where the engine stalls or cannot be started, conventional controls are generally incapable of functioning at all under such conditions and may even pose a danger of damage to or destruction of various parts of the transmission. As an example of this a partial loss in hydraulic fluid pressure may result in the two different clutches within the planetary section of the transmission being partially engaged. Since one of the clutches attempts to couple the rotatable carrier of the planetary section to a stationary housing while the other clutch attempts to couple the carrier to a rotatable drive shaft within the transmission, frictional elements within the clutches may burn up as a result.
Conventional transmission controls typically fail to distinguish between different hydraulic fluid utilizing functions within the transmission on a priority basis. Consequently, a slight reduction in hydraulic fluid pressure may cause a safety device to completely cut off the flow of hydraulic fluid to all parts of the transmission. Not only is this dangerous in terms of damage to or destruction of certain parts which may malfunction upon partial pressure loss or when pressure is removed too quickly but it ignores the fact that various different portions of the transmission vary greatly in terms of their need for full or partial hydraulic fluid pressure to continue operating safely or normally.
Conventional transmission controls additionally lack certain features which would make them safer or more useful including such things as efficient and effective low pressure warning devices and arrangements capable of automatically shifting a control into a different range or condition upon partial or complete loss of hydraulic fluid pressure.
Accordingly, it is an object of the invention to provide an improved hydraulically operated transmission control.
It is a further object of the invention to provide a transmission control capable of conserving hydraulic fluid pressure upon a reduction in pressure at the source and of applying the remaining fluid to various different portions of a transmission on a priority basis.
It is a further object of the invention to provide a transmission control in which hydraulic fluid may be used to maintain a control for the transmission in a given condition so that the control is readily changed to provide a different condition upon reduction or loss of the hydraulic fluid pressure.
It is a still further object of the invention to provide a transmission control in which a portion of the transmission can be controlled by a valve arrangement which automatically changes the control portion of the transmission to a different condition upon reduction or loss in hydraulic fluid pressure.